Secure communication system

ABSTRACT

A secure communication system uses narrow beam laser transmissions from a relay station high above the surface of the earth and another station which may be a submerged submarine. Position reporting by a laser transmission from the other station or submarine to the relay station provides for acquisition, tracking and data transmission between the stations. For strategic use transmissions to the submarine for one way data transmission is achieved by radio relay to the relay station and laser down link transmission to the submarine. Transponding mode operation is provided for a two-way transmission data link.

This is a divisional of co-pending application Ser. No. 248,518 filed onMar. 21, 1981 which is a division of Ser. No. 206,766, filed July 2,1962; now U.S. Pat. No. 4,279,036.

This invention relates generally to communication systems and moreparticularly to a secure communication system capable of world-wide twoway communication with mobile vehicles.

The maintenance of an adequate secure communication link with nuclearpowered submarines as they range throughout the oceans of the world is aproblem of prime importance to present day military operations. Severalsevere requirements for such a communications link must be met. Inaddition to maintaining contact with the vehicle for commandinformation, it is of paramount importance that the present position ofthe vehicle not be disclosed to the enemy. Since the location anddestruction of a country's nuclear powered submarine forces will be aprimary objective in the event of actual armed conflict, ordinarycommunication systems cannot be employed.

Ordinary radio communications and underwater sound communication systemsare readily detectable and capable of being monitored with instrumentswhich will indicate the location of the source of the transmittedenergy, and hence are not considered secure for a submarine operating inenemy waters. Even the erection of a radio receiving antenna is fraughtwith a certain amount of danger since the waters in which a submarinemay be operating will likely be kept under radar surveillance and thepresence of an efficient radio receiving antenna above the surface ofthe water will be readily detectable.

The use of extremely low frequency radio waves has been resorted to asone solution for contacting submarines on a world-wide basis and thesesystems have the capability of contacting submarines submerged beneaththe surfaces of the sea. These low frequency systems are extremelylimited in bandwidth and hence information content that can betransmitted in a given interval of time. Due to these limitations,cryptographic code transmissions are hampered and the possibility ofselective calling is limited. Furthermore, the return message from asubmarine requires the erection of a surface antenna with the consequentpossibility of detection.

The present invention provides a system which maintains a substantiallycontinuous wide band communication link with any suitably equippedmobile station anywhere in the world and does so in such a way that theprobability of detection of the location of the mobile station isextremely remote. This system has the capability of communicating withsubmarines submerged under some favorable conditions and in any eventprovides for communication with submarines with nothing more than anundetectable surface unit required under the worst conditions. Thecommunication link is maintained without any continuous radiationemanating from the submarine and is established by a singleinstantaneous burst type transmission from the submarine which isconfined to such a narrow beam that its detection and short durationmake the probability of loss of security almost negligible.

The present inention provides a communication system which uses anestablished set of relay stations which preferably will be orbitingearth satellites of either stationary orbit or in a polar orbit withknown paths so that present position is always predictable to both theshort base or command station and the mobile vehicle with which it isrequired to communicate. Preferably the system of relays is provided bya suitable system of orbiting earth satellites, but in a given tacticalsituation a relay station of a temporary nature may be launched forrelaying messages during a limited interval of time while the tacticalsituation is in existance. The description of the present invention willproceed, however, with reference to a system of earth satellitescarrying equipment to be herein described. The number of satellitesrequired and their relative positions will be determined by the range ofthe equipment employed and the periodicity with which contact isrequired to be made. For example, if only periodic contact is required,a single orbiting satellite in polar orbit would be sufficient to makecontact between the mobile vehicle and a fixed station several timeseach day. On the other hand, if continuous contact is required as may benecessary for the split second decisions and communications required inthe event of nuclear war, a system of satellites may be employed suchthat a given mobile vehicle will always have at least one satellitevisible from its location anywhere on earth.

In accordance with the present invention, the satellite or other relaystation is employed to contact a submarine or other vehicle on theearth's surface or beneath the sea and relay messages to and from thesubmarine or other mobile vehicle from a fixed command station. For thispurpose the relay may employ any form of communication link, includingconventional radio communications, between the fixed station and thesatellite with the communication being made preferably in code andsuitable crytographic techniques employed to maintain the message secureeven though it is broadcast by radio transmission. Since the advent ofphased array radar systems, it will be within the capability of anyenemy to locate the position of all satellites orbiting the earth andhence, the presence of such a communications relay satellite must beassumed to be disclosed to the enemy and not relied upon for security inmaintaining communications. In order to employ such a known satellitefor secure communications with the submarine, therefore, it will benecessary to conduct the communication with the submarine withoutdisclosing the submarine's position. For this purpose the presentinvention employs a laser to communicate over an optical beam of narrowbeam width which transmits coherent light and employs broad-bandmodulation so that pulse transmissions of brief duration can transmitmessage of high information content.

To communicate with an optical beam, such as produced by a laser andsuitable optics, it is necessary to orient the beam in the direction ofthe intended receiver in order that the beam can be received at thereceiver . Under these conditions the perception of a beam which isdirected to the submarine with which it is desired to communicatepresents the possibility of detecting the submarine's position. In orderto avoid the correlation of beam position with submarine position,should the beams from the satellite laser be detected, the presentinvention provides for a plurality of transmissions over a volume scanwith false messages modulated on each periodic transmission to randompoints in the volume scan so that the enemy is confronted with a largenumber of transmissions having no identifiable characteristics todistinguish those which are false from the beam actually pointing at asubmarine. The false beams are used only for the purpose of camouflagingthe one or more true angular directed beams which actually are pointedto one or more submarines. Since the periodicity of these decoytransmissions throughout the volume scanned can be made large, thefeasibility of an enemy checking out all of the beam transmissions todetermined if a projected line along the beam would actually lead to asubmarine receiver would be completely impracticable.

In order to communicate by means of laser beams, it is necessary thatthe beam be directed at the receiver, as previously stated. Theprincipal problem in communication between a satellite and a submarinetherefore is to supply the satellite with information which would permitthe laser beam from the satellite to be directed toward the submarine.For this purpose the present invention employs a transmission from thesubmarine in the form of a single narrow beam burst from a laser sourcewhich is of such short duration and narrow beam width and so sporadic inrepetition that the risk that the submarine will be detected by theinterception by an enemy of this transmission is all but negligible. Thepurpose of the laser beam transmission from the submarine to thesatellite is to provide in the satellite in a single instant theinformation of the submarine's location for use in orienting the laserbeam from the satellite to point toward the submarine. Once thesubmarine has supplied this information by a single optical burst oftransmission received at the satellite, no further transmission from thesubmarine is required since this information is utilized in thesatellite to orient the laser beam from the satellite to illuminate thesubmarine optical receiver and to track the submarine as the satellitecontinues in orbit. Thus no further transmission from the submarine isrequired until either the submarine itself changes position on thesurface or beneath the sea or in the event that the submarine desires totransmit a message for relay by the satellite. Such transmissions may bemade in a short time interval due to the wide bandwidth the opticallaser provides thus permitting large information content in shorttransmission times.

In employing the system of the present invention, various wellknowntechniques can be employed to enhance the security of the system. Forexample, the various transmissions and the coding employed to preventunauthorized access to the ssytem can be programed on a suitablecryptographic basis which is known to a friendly submarine and afriendly shore station and programed into the satellite so that thebenefit of a definitely programed cryptographic sequence can be utilizedto avoid compromising the system. It will further be possible for thesubmarine to have precise information about the orbit of the relaysatellites employed and therefore it will be possible, since thesubmarine in general knows its own position quite accurately, for thelaser transmission from the submarine to the satellite to beaccomplished with a single transmission. The information about the orbitof the satellite and the submarine's own accurately known positionshould be known with sufficient accuracy to aim the submarine's laserbeam to intercept the satellite.

The transmissions from the satellite over the volume scanned aremodulated by dummy messages in all instances except those in which thebeam is directed to a submarine receiver in which an actual message isto be sent. Hence to an unauthorized receptor of such messages, nocharacteristic can be detected which will distinguish a true messagebeam from a false one. This may be accomplished by suitably encodingboth the dummy and real messages in a coded transmission which makes thefalse message appear to be as plausible a transmission as the truetransmission code sequence.

In order to maintain a communication link once the submarine has emittedthe tracking laser beam and the satellite laser has found the locationof the submarine with the aid of this single transmission therefrom, atrack computer is employed in the satellite which utilizes inertial dataor data relating to its own orbit introduced from the ground stationwhich permits the laser beam transmitting the true message to track thesubmarine in angular orientation as the satellite proceeds on its orbit.This tracking of the submarine's position from the satellite canobviously be scheduled to be maintained for a period corresponding tothe accuracy of the tracking system or can be automatically terminatedby the time the satellite has moved to a position from which laser beamtransmission to the submarine is unfavorable, such as the position atwhich the satellite would be low on the horizon from the submarine'sposition.

The operational techniques for the submarine using the present inventionrequire that the submarine periodically contact one of the satelliterelays with its laser beam, which contact would complete thecommunication link with the shore station by transmitting to the shorestation the information that the link had been completed and give theshore station the location of the submarine. This position informationis read out of a storage device on the satellite which stores theangular position of the submarine for contacting the submarine byorienting the satellite laser beam and maintaining tracking of theangular position of the submarine as previously described. Once thecommunication link has been completed, the silence on the part of thesubmarine insures the security of the system and only when the submarineitself moves to a new location or when the satellite has moved in orbitto a position from which communication with the submarine is no longerpossible need the submarine transmit any further energy. The requirementfor the submarine to repeat its transmission to maintain contact with asatellite relay on either a periodic basis or on a continuous basis willbe determined by operational considerations.

The extremely narrow beams transmitted by lasers constitute a featurecontributing to the security of the system of the present invention. Asa practical matter, however, in view of the equipment required forinitial orientation of these beams and the tracking of the beams asrelative motion occurs when the satellite moves in orbit, it in allprobability will be necessary to use somewhat divergent laser beamsobtained through the use of suitable optics rather than the extremelyparallel beams of which the laser is capable. To the extent thatdivergent beams are required by the angular accuracy with which theorientation systems are able to operate, the detectability of the beamswill be increased with respect to unauthorized interception of the beamby the enemy observers. The angular accuracy capable of being built intothe system, however, should be such that the beams can be oriented inthe proper position for making the system operative with the employmentof beams which are considerably less divergent than the narrowest radarbeams, and hence the detectability of the beams to anyone who isslightly off axis should still present considerable difficulty and thusenhance security in the system.

It is the object of the present invention to provide a communicationsystem embodying the foregoing features for the purpose of providingsecure communications with a submarine or other mobile vehicles. Theseand other objects and features of the invention will be understood fromthe following detailed description taken in conjunction with theaccompanying drawings wherein:

FIG. 1 is a block diagram of the equipment present in a satellite asemployed in the invention; and

FIG. 2 is a block diagram of the equipment aboard the submarine inaccordance with the invention.

Referring now to FIG. 1, the equipment contained in the orbitingsatellite in accordance with the invention will be described. Theessential communication link between the satellite and the submarine isprovided by means of a laser 11 operating in the optical wavelengths inthe vicinity of visible light and preferably at a selected wavelengthwhich provides a maximum penetration of the atmosphere including watervapor. Penetration of cloud cover will require selection of a wavelengthin one of the transmission bands which are generally in the infra redregion of the spectrum. This criterion does not coincide with maximumpenetration of sea water for making contact with the submarine whilesubmerged. However, the use by the submarine of a surface periscopesupport for its optical transmission and reception elements will berelatively undetectable and hence the choice of wavelength for the laserwill be made to assure propagation during overcast clouds and rain. Seawater penetration could be achieved, if desired, by proper laserwavelength selection in the visible portion of the spectrum. Bothcapabilities could be combined in a system which used both lasers and acloud cover detector to select the appropriate one of the two lasers atany particular time.

The laser 11 emits a beam 12 of coherent light which is directed throughan optical system 13 that is adapted to produce an area scan betweenpredetermined limits 14 as required. The optical system 13 may be anywellknown optical scanner device, suitable ones being capable ofproducing full hemispheric scans if required. Due to the nature of thegeometry of the problem here involved, the limits 14 of the optical scanwill generally be less than a hemisphere and may be set to conform toother system limitations. The optics 13 are scanned by a scan generator15 which is operating to traverse the area scanned periodically.

The laser 11 is operated on a pulse basis in response to pulses receivedfrom a pulse generator 16 with the pulse pattern generated by thegenerator 16 preferably being somewhat random in order to avoid anysignificance attaching to the occurrence of a pulse at any given time inthe pulse train. The pulse generator 16 is arranged to have asynchronizing input 17 which when energized produces a pulse output thatwill energize the laser 11 and produce a beam 12 for transmissionthrough the optics 13 at the present scan position within the limits 14.

The normal operation of the system when contact is not completed to anysubmarine involves only those components just described and consists ofthe periodic area scan of the optics 13 with the random transmission ofa laser beam through the optics in accordance with the triggering of thelaser 11 by the random pulse pattern output of the generator 16.Alternatively, the pulse generator 16 can be arranged to produce anoutput to trigger the laser 11 only when a synchronizing input on line17 occurs. This mode of operation would be a transponding mode sinceeach laser beam would result from a synchronizing pulse derived from asignal received from a submarine as will hereinafter be described. Thistransponding mode would have the advantage of relatively infrequentlaser beams emanating from the satellite but would have the disadvantagethat each beam so emanating would be identified with the location of asubmarine.

In order to complete the communication link between the satellite andsubmarine, it is necessary for a transmission from the submarine to bereceived to aid the satellite in its tracking of the submarine. For thispurpose a wide angle optical system 21 is provided for a two-dimensionalarray of detectors 22 to provide a continuous surveillance system of thesurface of the earth corresponding to the area scan encompassed betweenthe limits 14 of the optical scanning device 13. The detector array 22is indicated only schematically and will in general be a fine resolutionarray of photoconductors which are capable of responding to a laserlight beam with the optical system 21 imaging the projected area of theearth on the array 22 which will permit the required degree of angularresolution by the array 22. Since narrow beam laser transmission is oneof the features of the invention relied upon to achieve security, itwill be necessary to provide an accurate angular resolution by means ofthe array 22 and for this reason the array 22 may be constructed of avery large number of extremely small photo responsive elemental areas.Such an array will also have an appriciable overall size in order thatthe energization of one of the elemental areas by a laser beam passingthrough the optical system 21 may be interpreted as an angular locationfor the origin of that beam from the earth with the desired degree ofaccuracy. As previously stated, the ultimate beam width of lasertransmission will in all probability not be capable of being utilizeddue to the mechanical limitations of angular orientation of the scanningsystems and the limitation on angular resolution which can be obtainedin a practical manner from a photoconductor array 22. Accordingly, theresolution of the array 22 may be made compatible with the beam width ofthe laser transmission employed by using suitably divergent beamsobtained by means of the laser optics.

The photo responsive array 22 is connected to a suitable matrix 23 whichis adapted to transmit position signals corresponding to the energizedphoto responsive area of the array 22 upon the energization of acoincidence input 24 to the matrix 23. The signal resolved by the array22 and converted by the matrix 23 is placed into a storage unit 25whenever the coincidence input 24 of the matrix is energized with theoccurrence of a resolved signal from the array 22. In the storageelement 25 the position information may be in any suitable form, such asany binary coded storage signal.

The purpose of the coincidence input 24 of the matrix 23 is to preventjamming of the system by a continuous illumination or other jamming andnoise signals received through the optical system 21. To achieve thisthe coincidence input 24 is energized from a decoder 26 which is fedfrom an optical receiver 27. The optical receiver 27 receives opticalsignals through a suitable system of optics 28 which is observing thesame field of view as the optical system 21 and the scan area traversedby the optical system 13. The transmission received by the opticalreceiver 27 from the submarine is a coded transmission which willsatisfy the decoder 26 that a true message from a friendly submarine isbeing received and thus produce on line 24 the required time coincidencesignal for the matrix 23.

The optical receiver 27 may be any suitable device for receiving lasertransmissions. For example, a laser photodetector driving a wide bandtraveling wave tube amplifier or an optical superheterodyne receiveremploying a laser local oscillator for demodulating the lasertransmissions and recovering the extremely broad band modulation ofwhich laser transmitters are capable will be suitable. With this broadband communication capability, the time intervals for the transmissionof any given message content can be made extremely small therebyenhancing the security of the transmission and making cryptographicprocedures feasible.

The storage device 25 is arranged to supply its output signal in theform of a binary coded word indicating angular position to a trackcomputer 31 which upon the receipt of a signal begins to compute presentangular position corresponding to the initial angular position inputsignal to compensate for the motion of the satellite relative to thesurface of the earth. This present angular position information isapplied on line 32 to the storage unit 25 to up-date the presentposition angular information contained in storage. Obviously, thestorage device 25 and the track computer 31 can be arranged as multiplechannel devices which are capable of exchanging these signals on amultiple basis to accommodate tracking the present angular position ofmore than one submarine. Thus if the array 22 is energized by truesignals from more than one friendly submarine at different locationswithin the field of view of the optical systems involved, all of thesepositional signals can be put into a multiple channel storage 25 andprocessed sequentially by the track computer 31 to maintain the presentangular positions up-dated for as long as desired.

The output of the track computer 31 is applied to a comparator 33 ascontinuous binary inputs of the present angular position data for allsubmarines which are currently being tracked and stored in the units 25and 31. A similar angular position binary signal is derived from thescan unit 15 on line 24 and applied to the comparator device 33 so thatan output pulse is obtained on line 35 whenever the scan position 15coincides with any present position angular data applied from the trackcomputer 31 to the comparator 33. This output pulse on line 35 isapplied as a synchronizing signal on input 17 of the pulse generator 16and therefore triggers the laser 11 to radiate a laser beam at theorientation then provided by the scan optics 13. In this fashion thesignal transmitted from the submarine and received through the optics 21and array 22 and decoded by means of the matrix 23 with the aid of thedecoding signal establishes one of the transmission beams from the laser11 at an angular orientation which will illuminate the submarine and,hence, complete the communication link to the submarine from thesatellite. By virtue of the up-dating of angular position data providedby the track computer 31, this synchronization of the laser 11 willoccur for each scan cycle produced by the scan device 15 at a time whenthe beam is directed toward the submarine irrespective of the motion ofthe satellite above the surface of the earth and hence tracking of thesubmarine will be good as long as the submarine is stationary within thelimits of the beam width transmitted.

The communication link via the laser beam through the optics 13 to thesubmarine is adapted to be modulated with a coded message to providedata link transmission and for this purpose a suitable modular 37 isconnected to modulate the laser 11. The normal message content providedby the modulator 37 is obtained from a dummy message unit 38 which isconnected to be synchronized by the pulse outputs of the generator 16and, therefore, supply message content input to the modulator 37 whichis applied as modulation to the laser 11 for all dummy transmissions.For this purpose a switch unit 39 is arranged to normally connect thedummy message unit 38 to the input of the modulator 37.

Upon the occurrence of an output from the comparator device 33 on line35, a connection 41 to the switch 39 changes the state of the switch 39to connect a buffer 42 to the input of the modulator 37 and disconnectthe dummy message unit 38 from modulator 37. The same pulse on line 35from the comparator device 33 is also applied to the buffer storage unit42 to read out of storage whatever message may have been placed in thebuffer 42 prior to the time of occurrence of the pulse on line 35. Thusthe output of the comparator device 33 switches the input of themodulator 37 from the dummy unit 38 to the buffer 42 and reads out ofthe buffer 42 any messages contained therein, as modulation for laser11. In this fashion the laser beam which is selected to be transmittedat the time the optics 13 is oriented to illuminate the submarine willalso be modulated with the message from the buffer 42. The buffer 42 maycontain a residual dummy message for modulating the laser 11 in theabsence of a current real message to avoid the transmission of atell-tale unmodulated beam.

For the purpose of completing the communication data link with a shorestation or other remote station on or above the surface of the earth, aradio relay 43 is provided having a suitable antenna system 44 forconventional data link type transmission between the earth and thesatellite. Messages received from the command station on the earth bythe radio relay 43 are placed in buffer storage 42 for transmission tothe submarine at the next transmission which will illuminate therecipient submarine. For this purpose a suitable address may be added tothe message which can be arranged to be rejected by all but theaddressed receiver submarine to effect selective call. Alternativearrangements for selective call are possible and may, in fact, bepreferable to avoid the repeated transmission of a true message fordetection and deciphering by the enemy. Suitable arrangements for usingan address code as received on the optical receiver 27 to channeladdressed messages from the radio relay 43 into a multi-channel buffer42 with read out from the multiple channels in buffer 42 being selectivein accordance with the scan location of the unit 15 corresponding to theaddressed submarine can readily be provided by those skilled in the art.With an arrangement of this type, the multi-submarine capability of thesystem will be achieved with a transmission of the addressed messagesoccurring only once, after which the individual channel can be clearedof the message but retaining the address awaiting the receipt of thenext message addressed to that particular submarine.

The communication link from the submarine to the satellite is completedby means of a laser beam received by the optical receiver 27, aspreviously described. In addition to decoding this message for thepurpose of transferring signals from matrix 23 into storage 25, thedecoder 26 may be employed to transmit true messages into a buffer 45.The buffer 45 also is arranged to receive on line 30 the angularinformation data signal that is present in the storage unit 25 andcontinuously up-dated by means of the track computer 31. The signal inbuffer 45 may, therefore, include an address identifying the submarinefrom which the communication has been received, a message transmitted bythe submarine, and further characterized by the angular position of thatsubmarine from the satellite. The information from the buffer 45 istransmitted by the radio relay 43 to the ground station whenever theradio relay is free for access by the buffer 45, i.e., whenever it isnot receiving transmissions from the ground if a single channel radiolink is employed. Manifestly, the buffer 45 may be a multiple channelstorage unit with plural submarines accommodated by a plurality ofchannels available for the address, message and angular positionsignals.

Referring to FIG. 2, the equipment required aboard the submarine orother mobile vehicle to complete the system will be described. Anoptical system 51 is adapted to receive laser beam transmissions from apredetermined field of view for application to an optical receiver 52which may be of the same general type as the receiver 27. Signalsreceived by the optical receiver 52 are decoded by a decoder 53 which isadapted to process the security code employed and pass the messagecontent into storage 54 from which it may be read out in anyconventional read-out device 55.

For the purpose of initiating the tracking operation in the satellite, anarrow laser beam transmission from the submarine is required inaccordance with the invention. For this purpose a laser 56 is providedwith a suitable optical system 57 which is capable of directing thelaser beam in any desired angular orientation. A modulator 58 isprovided to modulate message content on to the laser transmission and apulse generator 59 is provided to trigger the transmission burst fromthe laser 56. The transmission sequence can be initiated by applying asignal on an input 61 to the modulator 58 for applying identifyingaddress and code modulation to the laser 56 as it is triggered by thepulse generator 59 which is triggered by the output of the modulator 58.

The orientation of the beam provided by the optical system 57 iscontrolled by a tracker unit 62 which has a manual input 63 forestablishing an initial orientation of the beam 64 emanating from theoptics 57. Since the submarine has precise information as to its ownpresent position and will be aware of the established orbit of thesatellite, the manual input 63 can be employed to orient the beam 64 toan angular position which will illuminate the satellite for the singletransmission burst initiated by an input to the modulator 58 on lead 61.Tracking the satellite from the submarine is completed upon the receiptof a laser beam by the optical receiver 52 which when decoded is appliedvia lead 65 to the tracker 62 for the purpose of operating the tracker62 and optical system 57 in the track mode to track the orbit of thesatellite. This tracking may be on the basis of the pre-computedsatellite orbit as a function of time and the present position of thesubmarine or it may be with the aid of angular position informationtransmitted from the satellite and received by the submarine in receiver52. For this purpose the angular position of the optics 13 in thesatellite are modulated on to the transmission from laser 11 in additionto other message content giving the polar cordinates of the line ofsight between the satellite and the submarine which polar cordinates areapplied on line 65 to the tracker 62 for correcting the angularorientation of the optics 57 thereby assuring illumination of thesatellite whenever a subsequent transmission by the laser 56 occurs. Itwill be appreciated that once tracking is established preferably by asingle burst transmission from the laser 56 no further transmission fromthe submarine will occur unless an actual message content signal is tobe transmitted via the satellite to the command post shore station whichmessage may be applied at the input 61 to the modulator 58. In this modeof operation, in order to maintain security of the submarine, no suchtransmissions will be made unless absolutely necessary and the onlytransmission which the submarine will be required to complete is asingle burst for tracking purposes whenever the submarine changes itsposition by an amount greater than the beam width of the transmissioninvolved or the satellite passes out of range and a new satellite isselected by the submarine for maintaining the communication link.

The apparatus of FIG. 2 may be carried aboard the submarine withsuitable portions thereof available on a periscope type mount in orderthat the optical elements may be raised to the surface of the sea underadverse conditions. Whenever the angle of transmission to the satelliteis such that an unfavorable angle of incidence to the surface of the seaexists, penetration of the laser beam transmission from the satellitecannot be relied upon and the erection of the optical elements to thesurface of the sea may be required. Under sea state conditions when thesurface of the sea presents a rapidly varying interface angle due towave action, the surface optical elements may also be used. Underconditions of favorable angle of incidence in relatively calm seas,however, the optical transmissions involved may be relied upon topenetrate the sea water to a certain extent and the submarine may,therefore, communicate without the possibility of disclosing itsposition by any periscope projection to the surface of the sea.

The signal on line 65 which is periodically received at the rate of thearea scan provided by the unit 15 of the satellite may be employed as atrigger signal to the input 61 of the modulator 58 whenever the trackingsignal from the satellite is not received. For this purpose anintegrator unit 66 is energized by the periodic output on line 65 toproduce no output as long as the periodic signals having the period ofthe scan unit 15 in the satellite are received. If these signals shouldbe lost, however, the unit 66 produces an output to the input 61 of themodulator 58 thereby producing a laser burst output beam 64 whichsupplies the satellite with a new tracking aid signal in order tore-establish tracking from the satellite. Obviously, this sequence willhave to be perfected while the satellite is still within the beam widthof the beam 64. If difficulty is encountered in intercepting thesatellite with the beam 64 under any conditions, it may be expedient toprovide the optical system 57 with a control 67 which will make the beam64 more widely divergent during search mode in order to secure aninterception of the beam 64 with the satellite.

Many refinements in the system which are conventional techniques inmodern day astronautic and communication systems have not been mentionedbut will be recognized as requirements by those skilled in the art. Forexample, in order for the satellite to accomplish tracking ascontemplated by the invention, a stabilization system will be requiredand a stable vertical reference may be achieved by suitable inertialsystems or an horizon scanner or both. Ordinarily, a sufficiently stableorbit for the satellite can be achieved to permit an accurate predictionof present position of the satellite in order for the submarine toinitiate tracking with a minimum amount of radiation emanating from thesubmarine. If this proves to be a difficult criterion to meet, asuitable continuous radio or infra red tracking signal may be radiatedfrom the satellite for permitting the submarine to locate the satellitewith sufficient angular accuracy to make the requisite transmission ofits laser beam on a short term and highly sporadic basis as contemplatedherein.

In situations where maximum security is not a primary factor, the systemcan be readily modified to permit a transponding mode between the lasersin the satellite and the ground station. For example, each satellitelaser transmission would be triggered by the receipt of a lasertransmission from the ground station and vice-versa to maintain dataexchange therebetween. For these systems angle tracking of the beamswould be simplified by introducing well-known conical scan techniquesand transponding off-axis error signals to servo the scan axis.

In order to avoid characterizing the tracking beam from the satellite byvirtue of the fact that, while tracking, the beam illuminating asubmarine is stationary on the surface of the earth, the false beams canalso be arrested in angular position to present a stationary ensemble ofbeam positions.

While the invention has been described for use in communicating with asubmarine, it will be apparent that the broad features of the inventioncan be more widely applied.

I claim:
 1. The method of communicating with a mobile vehicle comprisingthe steps of transmitting a laser beam toward a relay station high abovethe surface of the earth from a vehicle within the portion of theearth's surface beneath said relay station, detecting said laser beamreceived at said relay station to determine the angular orientation ofsaid vehicle from said relay station, utilizing the angular orientationdata so obtained to establish a narrow beam communication link from saidrelay station to said vehicle, and communicating with said vehicle froma remote point via said relay station and said narrow beam communicationlink.
 2. The method of communicating with a mobile vehicle comprisingthe steps of scanning a portion of the earth's surface with an opticalspot scan from a relay station high above the surface of the earth,transmitting a laser beam toward said relay station from a vehiclewithin said portion of the earth's surface, detecting said laser beamreceived at said relay station to determine the angular orientation ofsaid vehicle from said relay station, utilizing the angular orientationdata so obtained to transmit a laser beam from said relay stationthrough said optical scan when said scan is oriented toward saidvehicle, detecting the laser beam received at said vehicle, andcommunicating with said vehicle from a remote point by messagesexchanged between said remote point and said relay station with saidmessages being exchanged between said relay station and said vehicle bymodulation of the laser beams transmitted and demodulation of the laserbeams received.
 3. The method according to claim 2 in which the laserbeam from said relay station is repeatedly transmitted throughout saidoptical scan to produce a plurality of laser beams of different angularorientation emanating from said relay station, all of said beams beingmodulated and indistinguishable from each other except for the messagecontent of the laser beam oriented toward said vehicle.
 4. The method ofestablishing a communication link via a relay station high above thesurface of the earth with a narrow beam energy transmission from aground station and utilizing the received energy transmission at therelay station to orient a narrow laser beam energy transmission from therelay station to the ground station to complete the communication linkbetween the relay station and ground station.
 5. The method according toclaim 4 in which said narrow beam transmissions are laser beamsoperating at a wavelength in the infra-red portion of the spectrumselected for transmission through the atmosphere containing water vapor.6. The method according to claim 4 in which said narrow beamtransmissions are laser beams operating at a wavelength in theblue-green portion of the spectrum selected for transmission through seawater.
 7. The method of communicating with a first station comprisingthe steps of transmitting a laser beam toward a relay station high abovethe surface of the earth from said first station, detecting said laserbeam at said relay station to determine the angular orientation of saidfirst station from said relay station, utilizing the angular orientationdata so obtained to establish a narrow beam communication link from saidrelay station to said first station and communicating with said firststation from a remote point via said relay station and said narrow beamcommunication link.
 8. The method of claim 7 wherein said narrow beamcommunication link is a laser beam.
 9. The method of communicating witha submarine comprising the steps of scanning an area beneath a relaystation high above the surface of the earth to cover the expectedlocation of the submarine, receiving a message on said relay stationfrom a ground station and transmitting a laser beam communication ofsaid message from said relay station to said submarine.
 10. The methodof communicating with an earth station comprising the steps of relayinga message from a ground station to a relay station high above thesurface of the earth, modulating a laser on board said relay stationwith said message to produce a modulated laser beam, radiating saidmodulated laser beam from said relay stdation to the surface area of theearth where said earth station is located, and detecting the message atsaid earth station from said modulated laser beam received at said earthstation.
 11. The method according to claim 10 wherein said laser beamoperates in the blue-green portion of the spectrum selected fortransmission through sea water and said earth station is a submarineadapted to detect said message from said laser beam while submerged. 12.The method according to claim 11, wherein said modulated laser beam isscanned over the area of the ocean where said submarine is located. 13.The method according to claim 9 wherein said laser beam operates in theblue-green portion of the spectrum selected for transmission through seawater and said submarine is adapted to detect said message from saidlaser beam while submerged.
 14. A communication station comprising:avehicle operable high above the surface of the earth having on board:aradio receiver for receiving and demodulating a radio signal transmittedto said vehicle from a first remote station to obtain the messagecontent of said radio signal; laser transmitter means operable fortransmitting a laser beam from siad vehicle; means for modulating thetransmitted laser beam with said message content; and means fordirecting the modulated laser beam toward a second remote station totransmit said message content to said second remote station.